Comment by Galanwe
8 hours ago
It's not that the borrow checker is too difficult, it's that it's too limiting.
The _static_ borrow checker can only check what is _statically_ verifiable, which is but a subset of valid programs. There are few things more frustrating than doing something you know is correct, but that you cannot express in your language.
For kernels (and I suspect database engines might be added to the list, since they seem to have similar requirements to be both scalable and deal with massive amounts of shared state, but I'm not overly familiar with them) is where it gets particularly difficult.
Several kernels for example use type-stable memory, memory that is guaranteed to only hold objects of a particular type, though perhaps only providing that guarantee for as long as you hold an RCU read-lock (this is the case in Linux with SLAB_TYPESAFE_BY_RCU). It is possible in some cases to be able to safely deal with references to objects where the "lifetime" of the referent has ended, but where by dint of it being guaranteed to be the same type of object, you can still do what you want to do.
This comes in handy when you have a problem that commonly appears in kernels where you need to invert a typical lock ordering (a classic case is that the page fault codepath might want to lock, say, VM object then page queue, but the page-replacement codepath will want to lock page-queue then VM object.)
Unfortunately it's hard to think of how the preconditions for these tricks could be formally expressed.